Web guide apparatus and method

ABSTRACT

A web guide apparatus and method in which the lateral position of a traveling web passing over a tiltable web guide roller is sensed at a location in advance of the tiltable web guide roller and the web leaving the tiltable web guide roller is caused to double back to a point adjacent a plane through the web edgesensing location and normal to the plane of tilting movement of the web guide roller. The web is guided so as to enter the tiling web guide roller in a plane generally paralleling the plane of motion of the web guide roller.

United States Patent Inventor Robert D. Rule Rockford, Ill.

Appl. No, 799,197

Filed Feb. I4, 1969 Patented June l, 1971 Assignee Rockford Servo Corporation Rockford, Ill.

WEB GUIDE APPARATUS AND METHOD I 56] References Cited UNITED STATES PATENTS 3.4l 1,683 ll/l968 Battles etal, 226/21 Primary Examiner-Allen N. Knowles Att0rney-Mc Canna, Morsbach, Pillote & Muir ABSTRACT: A web guide apparatus and method in which the lateral position of a traveling web passing over a tiltable web guide roller is sensed at a location in advance of the tiltable web guide roller and the web leaving the tiltable web guide roller is caused to double back to a point adjacent a plane through the web edge-sensing location and normal to the plane of tilting movement of the web guide roller. The web is guided so as to enter the tiling web guide roller in a plane generally paralleling the plane of motion of the web guide roller.

WEB GUIDE APPARATUS AND METHOD BACKGROUND'OF THE INVENTION It is common practice in guiding traveling webs intermediate therein to pass the traveling web over a web guide apparatus mounted for tilting movement relative to the pass of web travel and to sense the lateral position of the web and automatically tilt the web guide apparatus in a direction to correct the deviation of the web from the desired path of web travel. It is generally desirable to sense the position of the traveling web as it leaves the web guide apparatus so that the sensor responds to the corrected path of web travel produced by the web guide apparatus. However, in some installations, location of the web edge sensor at the exit side of the web guide introduces other problems. For example, when the web leaves the web guide apparatus at a substantial angle to the plane of motion of the web guide apparatus, the exiting web is not only shifted laterally be the web guide apparatus in the plane of the exiting web to correct the lateral deviation of the web from the desired pass of web travel, but the web is also subjected to a shifting in a direction normal to the plane of the exiting web, which last-mentioned shifting of the web is herein referred to as pass-line fluctuation of the web. Substantial movement of the web in a direction normal to its plane markedly complicates the problem of sensing the position of the web. The sensor must not only be such as to physically accommodate the maximum pass-line fluctuation of the web, but must also produce a signal which is correlative with the lateral movement of the web in its plane, but substantially independent of the shifting movement of the web in a direction normal to its plane. Some sensors, such as gap-type web edge sensors, can accommodate only limited shifting of the web in a direction normal to its plane, and some others, such as vacuum-type sensors, can accommodate almost no shifting of the web normal to its plane.

In order to minimize the pass-line fluctuation at the web edge sensor, it has heretofore been common practice to locate the web edge sensor a substantial distance downstream of the exiting web guide roller. However, when the web guide is located downstream in slow speed web-handling system, the web guide must overcorrect or overshoot before the web edge sensor is satisfied, and this tends to cause hunting of the web guide.

SUMMARY OF THE INVENTION A traveling webis guided intermediate its ends by passing the web over a main web guide roller mounted for tilting movement in a plane of motion in relatively opposite directions from a neutral position; the lateral position of the web is sensed by sensing the position of the web entering the main, guide roller at a location in advance of the main guide roller, and the web exiting from the main guide roller is caused to double back over an exiting web guide roller so as to exit from the exiting web guide roller at a point which is adjacent a plane through the sensing location and normal to the plane of motion of the web guide roller. This arrangement substantially minimizes the triangulation error between the position of the web edge as sensed by the sensor and the position of the web edge as sensed by the sensor and the position of the web edge exiting from the web guide apparatus. The web entering the main guide roller is guided in a plane generallyparalleling the plane of motion of the main web guide roller so as to minimize pass-line fluctuation of the web entering the main web guide roller in the region of the web edge sensor.

An important object of this invention is to provide a web guide apparatus in which the lateral position of theweb edge is sensed at a location in advance of the tiltable web guide roller while minimizing triangulation error between the lateral position of the web edge as sensed by the sensor and the lateral position of the web edge exiting from the web guide apparatus.

Another object of this invention is to provide a web guide apparatus in accordance with the foregoing object, which also provides an S-wrap of the web over the rollers for improved web to roller traction.

Another object of this invention is to provide a web guide apparatus in which the web edge sensor is subjected to a minimum pass-line fluctuation.

These, together with other objects and advantages of this invention will be more readily understood by reference to the following detailed description when taken in connection with the accompanying drawings wherein:

FIG. 1 is a fragmentary perspective view of a web-handling apparatus embodying a web guide apparatus constructed in accordance with the present invention;

FIG. 2 is a side view taken on the plane 22 of FIG. 1;

FIG. 3 is a fragmentary end view taken on the plane 33 of FIG. 2;

FIG. 4 is a fragmentary view of a modified form of web guide apparatus embodying the present invention taken on the plane 4-4 of FIG. 5;

FIG. 5 is a fragmentary end view of the web guide apparatus of FIG. 4;

FIG. 6 is a diagrammatic view illustrating the triangulation error which occurs at different points in the web guide apparatus; and

FIG. 7 is a diagrammatic view illustrating the relationship of the web guide rollers and web edge sensor in the web guide of FIG. 6.

The web guide apparatus of the present invention is generally adapted for use in web-handling systems for laterally guiding a web W intermediate its ends and is shown in FIG. 1 applied to a web handling system having stationary guide rollers l0 and 11. The web guide apparatus is located to engage the web W intermediate the rollers 10 and 11 and is arranged to laterally shift the web as it passes from the roller 10 to the roller 11 in a manner to substantially maintain the web in the desired path of web travel. The web guide apparatus 12 in general includes a main web guide roller 15 supported for tilting movement in either direction from a neutral position generally normal to the desired path of web travel. In most applications, the rollers are disposed in rolling engagement with the web. However, in some applications, such as in some installations of tracking roller assemblies, the rollers can be stationary rods over which the web passes. The guide apparatus also includes a web edge sensor 16 located to sense the lateral position of the web at a location in advance of the main web guide roller 15 and a control apparatus 17 responsive to the signal produced by the sensor 16 for operating an actuator 18 for shifting the web guide apparatus in a direction to correct the deviation of the web from the desired path of web travel. The sensor 16 and control apparatus 17 can be constructed and arranged for either edge guiding of the web or centerline guiding of the web, as desired. Thus, if the web is laterally offset in one direction, such as to the right, of the desired path of travel, the control apparatus operates to tilt the roller 15 in the other direction, that is to the left, to correct the deviation of the web from the desired path of travel. The web guide apparatus also includes an exiting web guide roller 19 located in a manner described more fully hereinafter so as to cause the web passing from the main guide roller 15 to double back to a location so as to minimize the triangulation error between the web position as sensed by the sensor 16 a d the web position as it leaves the web guide apparatus.

Various different arrangements can be utilized to support the main guide roll 15 for tilting movement. In the embodiment shown in FIG. 1, the main web guide roller is supported so that, during tilting movement thereof, the main web guide roller is disposed generally tangent to an arcuate path having an effective are center designated P in FIG. 1 and located in advance of the main web guide roll. The mechanism shown for supporting the main web guide roll for tilting movement is of the type generally disclosed in the patent to Ott, Jr. U.S. Pat. No. 3,390,823, and which includes a plurality of arcuate guide bars 21 mounted on a stationary frame structure 22 and constructed and arranged so as to have a common are center at P. Slide members 24 are slidably mounted on each of the arcuate guide bars 21 and support a movable frame structure 23 for movement about the arc center P. The main web guide roll 15 is rotatably supported as by a shaft on the movable frame structure at a point radially spaced a substantial distance from the arc center P. With this arrangement, the main web guide roll 15 remains generally tangent to an arcuate path having an arc center at P so as to produce a combined longitudinal shifting and tilting of the guide roll as it moves through the arcuate path.

In order to have the web guide apparatus automatically respond to corrections in the path of web travel introduced by the web guide apparatus, it is generally desirable to sense the lateral position of the web as it exits from the web guide apparatus. However, when a web exits from a tiltable web guide roller in a direction transverse to the plane of motion of the web guide roller, the web is not only shifted laterally in the plane of the web by the tilting web guide roller, but is also shifted in a direction normal to the plane of the web. This lastmentioned shifting motion, in a direction normal to the plane of the web as it exits from the web guide roller is herein referred to as pass-line fluctuation of the web. The pass-line fluctuation varies as a function of the angle through which the guide roller is tilted and the width of the web guide roller. In many installations, particularly in those involving relatively wide webs or rollers which tilt through substantial angles, the pass-line fluctuation becomes quite large and makes it difficult to provide a sensor which is capable of accommodating the pass-line fluctuation, Moreover, as pointed out above, locating the web edge sensor downstream of the web guide apparatus increases the tendency of the web guide apparatus to hunt, particularly in slow speed web-handling systems.

In order to reduce the pass-line fluctuation at the web edge sensor, the sensor 16 is mounted so as to sense the lateral position of the web as it enters the main web guide roller 15 and at a location where the web is not subjected to substantial movement in a plane normal to the plane of the web. This is best achieved by guiding the web as it enters the main web guide roller in a plane generally parallel to the plane of motion of the main web guide roller. In the form shown in FIG. 1, an inlet web guide roller 26 is mounted on the movable frame structure 23 for movement therewith about the arc center P. The inlet web guide roller 26 is preferably so arranged that the arc center or pivot point of the web guide apparatus 23 is disposed in the plane of the web as it passes from the roller onto the inlet roller 26 and the roller 26 is advantageously arranged in relation to the main web guide roller so that the plane of the web as it passes from the inlet roller 26 to the main web guide roller is disposed generally parallel to the plane of motion of the tiltable web guide roller. When the plane of motion of the web as it enters the roller 15 is exactly parallel to the plane of motion of the roller, there is theoretically no shifting of the entering web in a direction normal to the plane of the entering web. However, some web edge sensors can accommodate a small shifting of the web in a direction normal to the plane of the web and it is therefore not essential that the web be guided onto the main web guide roller in a plane exactly parallel to the plane of motion of the main web guide roller. The motion of the entering web in a direction nonnal to its lane, due to tilting movement of the main web guide roller 15, will not become substantial until the plane of the entering web is inclined substantially to a plane parallel to the plane of motion of the main web guide roller. Satisfactory results as regards minimizing pass-line fluctuation at the web edge sensor can accordingly be achieved when the plane of the entering web is only generally parallel to the plane of motion of the tiltable web guide roller and the entering plane may, for example, deviate by 10 or l5 from the plane of motion ofthe web guide roller without causing excessive pass-line fluctuation at the web edge sensor.

When the web edge sensor is positioned in advance of the tiltable roller from which the web exits from the guide apparatus, there is an error, herein sometimes referred to as a triangulation error, between the position of the web edge as sensed by the sensor, and the position of the web edge as it leaves the tiltable roller. This triangulation error is zero when the tiltable web guide roller is disposed perpendicular to the desired path of travel of the web and increases to the right and to the left as the rollers are respectively tilted toward the right and toward the left relative to the path of web travel. This triangulation error will be best understood by reference to FIGS. 6 and 7 wherein the main web guide roll 15 is diagrammatically shown supported for tilting movement about an arc center I As shown, the web edge sensor 16 is positioned so as to sense the lateral position of the web edge E, as it enters the main web guide roller. The web edge sensor 16 is spaced from the outer face 15 of the main web guide roller 15 a distance designated 2, which must be sufficiently large to accommodate the width of the roller 15, the left pass-line fluctuation designated 2,, and some additional working clearance. The radial spacing of the outer face 15' of the web guide roller 15 from the arc center 5 is designated as the throw 1. If the web L/ were caused to exit from the main web guide roller in a line tangent to the outer face 15' of the roller, it will be seen from FIG. 6 that, when the roller 15 is disposed perpendicular to the desired path of web travel, the lateral position of the web edge E as sensed by the sensor 16, will correspond to the lateral position of the web edge, as it passes over the outer face 15' of the roller 15. However, when the main web guide roller 15 is tilted through an angle A, the position of the web edge shown in dotted lines at IL, is sensed by the sensor 16 will differ from the lateral position of the web edge as it passes over the outer face 15 of the roller 15 by a right triangulation error designated 5, in FIG. 6. Conversely, when the web guide roller is tilted to the left as viewed in FIG. 6 through a corresponding angle 4, the position of the web edge designated W as sensed by the sensor 16 will differ from the web edge position as it passes over the outer face 15' of the roller 15 by a left triangulation error designated E These triangulation errors designated g, and Q, between the position of the web edge as sensed by the sensor 16 and the position of the web edge as it passes over the outer face 15 of the main roller 15 can be approximately expressed by the following equations:

(1) Approx E =D tan A+Z, tan A (2) Approx E, ='-tan A+Z tan A where positive signs indicate toward the right and negative signs toward the left.

The geometry of a web guide roller is sometimes expressed in terms ofsidelay, that is the maximum endwise movement of the web guide roller in a direction nonnal to the path of web travel, and the throw of the web guide roller, that is the distance between the exit point of the web from the roller and effective pivot point of the roller. If the web exited from the main web guide roller 15 along a line tangent to the outer face 15' of the roller 15, it will be seen from FIG. 6 that the sidelay would correspond to the distance designated by the letter S and the throw of the main web guide roller 15 would cor respond to the distance designated by the letter T. The maximum tangent A can be approximately defined in terms of sidelay and throw of the roller by the following equation:

(3) Max. approx tan A=Sl2T If the web exited from the main web guide roller 15 in a direction tangent to the outer face 15 of the main web guide roller, it will be seen from equations 1 and 2 that there would be a substantial triangulation error designa A Q and f when the web guide roller is tilted respectively to the right andto the left from its neutral position shown in solid lines in FIG. 6. From equations 1 and 2 it will also be st-en that this triangulation error includes a component D tan 4 which is a function of the spacing of the web edge sen? from the point where the web would exit from the roll 15, and another component 5 tan gwhich is a function of the pass-line fluctuation of the roll 15. Since the web edge sensor must be spaced from the outer edge of the roll 15 a distance sufficient to accommodate the width of the roller, the left pass-line fluctuation E together with some additional working clearance, it is apparent that the component 2 tan A is quite large as compared to the component 5 tan 4 Provision is made for substantially minimizing the triangulation error which occurs due to positioning of the web edge sensor in advance of the main web guide roll 15. For this purpose, the exiting web guide roller 19 is located in relation to the main web guide roller and the web edge sensor such that the web [passing from the main web guide roller over the roller 19 is caused to double back with relation to the web entering the main web guide roller 15 to a location at which the triangulation error is substantially minimized. A traveling web will tend to enter and leave a web guide roller in a direction normal to the axis of the roller. Assuming the web leaves web guide roller perpendicular to the axis of the roller, the exiting web guide roller should be located in the position shown in FIG. 3 in which the tangency point where the web 1 leaves the exiting roller 19 is disposed in a plane through the web edge sensor 16 and normal to the plane of motion of the main web guide roller 15. However, the tendency of the web to leave the roller in a direction perpendicular to the roller does vary with the stiffness of the web and the web speed. Thus, the position of the web edge sensor with relation to the tangency point of the exiting web might be varied slightly in some installations from the exact location shown and described, for optimum results. When the tangency point of the web as it leaves the last-moving web guide roller 19 of the web guide apparatus, is disposed in or approximately in a plane through the web sensing apparatus which is nonnal to the plane of motion of the web guide apparatus, this reduces the spacing between the web edge sensor and the point where the web edge leaves the web guide apparatus to substantially zero. Under these conditions, equations for the right and left hand triangulation error can be expressed approximately as follows:

(4) E,'=,' tan (5) E 40 =Z, tan 4 Thus, the triangulation error with the arrangement disclosed in FIGS. 1-3 is reduced to a relatively small value which varies as a function of the pass-line fluctuation.

The invention is shown applied to a modified form of web guide apparatus in FIGS. 4 and 5. In this embodiment, the web guide apparatus includes a main web guide roller 15' and an exiting web guide roller 19'. The main and exiting web guide rollers are mounted for tilting movement about an arc center 11 disposed in advance of the web guide apparatus, as by arcuate guide bars 21' and followers 24 as disclosed in the aforementioned patent to Ott, Jr. The web entering the main web guide roller 15 is guided over a stationary entering guide roller 26' diagrammatically illustrated in FIG. 5 and positioned so that the plane of the web entering the roller 15' is disposed generally parallel to the plane of motion of that roller. The web edge sensor 16' is located to sense the lateral position of the web entering the roller 15', so as to minimize pass-line fluctuation at the web edge sensor, and the exiting web guide roller is located with relation to the main web guide roller 15' and the sensing apparatus 16' so that the tan gency point of the web leaving the exiting roller 19' is disposed approximately in a plane through the web edge sensor 16' which is normal to the plane of motion of the web guide apparatus. With this arrangement, the effective spacing designated 2 is equations l and 2 between the location of the web edge sensor and the point where the web leaves the last roller of the web guide apparatus reduced to substantially zero. The remaining triangulation error is accordingly approximately that set forth in equations 4 and 5 above.

While the invention is shown applied to web guide apparatus having effective pivot points spaced a substantial distance in advance of the web guide roller, the invention is not limited to such web guide apparatus wherein the pivot point is spaced a substantial distance from the web guide roller and may, for. example, be applied to web guide apparatus in which the rollers tilt about an axis closely adjacent or even intersecting the axis of the web guide roller.

It is claimed:

1. A web guide apparatus including a main web guide roller extending transverse to a traveling web, means mounting the main web guide roller for tilting movement in a plane of mo-' tion generally paralleling the plane of the web as the latter enters the main guide roller in relatively opposite directions from a neutral position perpendicular to the desired path of web travel, at least one web edge sensor positioned to sense the lateral position of one edge of the web at a location in advance of the main guide roller, means controlled by said web edge sensor for tilting said main web guide roller in a direction to correct the deviation of the web from the desired path of web travel, and an exiting web guide roller mounted for tilting movement with the main web guide roller and positioned with relation to the main web guide roller and the web edge sensor such that the web passing from the main web guide roller over the exiting web guide roller is caused to double back with relation to web entering the main guide roller and exit tangentially from the exiting web guide roller in an exiting plane, the projection of which exiting plane extends approximately through said location of the web edge sensor.

2. In a web-handling system, a web guide apparatus including a main web guide roller extending transverse to a traveling web, means mounting the main web guide roller for tilting movement in plane of motion generally paralleling the plane of the web as the latter enters the main guide roller, a web edge sensor for sensing the lateral position of at least one edge of the traveling web, and means controlled by the web edge sensor for tilting the main web guide roller in a direction to correct the deviation of the web from the desired path of web travel, the improvement wherein the web edge sensor is positioned adjacent the plane of the web entering the main web guide roller at a location in advance of the main web guide roller to sense the lateral position of web entering the main guide roller, and an exiting web guide roller mounted for tilting movement with the main web guide roller and positioned with relation to the main web guide roller and the web edge sensor such that the web passing from the main web guide roller over the exiting web guide roller is caused to double back with relation to the web entering the main guide roller to a point adjacent a plane through the web edge sensor and normal to the plane of motion of the main web guide roller.

3. A web guide apparatus according to claim 2 wherein said main web guide roller during tilting movement thereof is disposed tangent to an arcuate path having an arc center located in advance of the main web guide roller and generally perpendicular to the plane of the web entering the main guide roller.

4. A web guide apparatus according to claim 3 including an entering web guide roller extending transverse to the web in rolling engagement therewith adjacent said are center and connected to said main web guide roller to move therewith about said arc center.

5. A web guide apparatus including a main web guide roller extending transverse to a traveling web, means including a movable support mounting said main web guide roller for movement in a plane of motion in which the main web guide roller is disposed generally tangent to a curved path, the effective center of which curved path is spaced from that side of main web guide roller at which the web enters the main web guide roller, at least one web edge sensor mounted to sense the lateral position of the edge of the web entering the main web guide roller at a location in advance of the main web guide roller, and an exiting web guide roller mounted on said movable support for movement with the main web guide roller and at a position ofiset relative to the main web guide roller in a direction toward the effective center of said curved path and such that the web passing from the main web guide roller and over the exiting web guide roller doubles back and exits tangentially from the exiting roller in an exiting plane, the projection of which exiting plane extends approximately through said location of the web edge sensor.

6. A web guide apparatus according to claim 5 wherein said plane of motion is generally parallel to the plane of the web entering said main web guide roller.

7. A web guide apparatus according to claim 5 including an entering web guide roller mounted on said movable support adjacent said effective center of said curved path for movement, said entering web guide roller being disposed in rolling engagement with the web and supporting the web entering said main web guide roller in a plane generally paralleling said plane of motion.

8. A method of laterally guiding a moving web comprising, passing the web over a main web guide roller tiltable in a plane of motion from a neutral position in which the main web guide roller is perpendicular to the desired path of web travel, sensing the lateral position of the web entering the main web guide roller at a sensing location in advance of the main web guide roller, tilting the main web guide roller in a direction to correct the deviation of the web from the desired path of 

1. A web guide apparatus including a main web guide roller extending transverse to a traveling web, means mounting the main web guide roller for tilting movement in a plane of motion generally paralleling the plane of the web as the latter enters the main guide roller in relatively opposite directions from a neutral position perpendicular to the desired path of web travel, at least one web edge sensor positioned to sense the lateral position of one edge of the web at a location in advance of the main guide roller, means controlled by said web edge sensor for tilting said main web guide roller in a direction to correct the deviation of the web from the desired path of web travel, and an exiting web guide roller mounted for tilting movement with the main web guide roller and positioned with relation to the main web guide roller and the web edge sensor such that the web passing from the main web guide roller over the exiting web guide roller is caused to double back with relation to web entering the main guide roller and exit tangentially from the exiting web guide rolLer in an exiting plane, the projection of which exiting plane extends approximately through said location of the web edge sensor.
 2. In a web-handling system, a web guide apparatus including a main web guide roller extending transverse to a traveling web, means mounting the main web guide roller for tilting movement in plane of motion generally paralleling the plane of the web as the latter enters the main guide roller, a web edge sensor for sensing the lateral position of at least one edge of the traveling web, and means controlled by the web edge sensor for tilting the main web guide roller in a direction to correct the deviation of the web from the desired path of web travel, the improvement wherein the web edge sensor is positioned adjacent the plane of the web entering the main web guide roller at a location in advance of the main web guide roller to sense the lateral position of web entering the main guide roller, and an exiting web guide roller mounted for tilting movement with the main web guide roller and positioned with relation to the main web guide roller and the web edge sensor such that the web passing from the main web guide roller over the exiting web guide roller is caused to double back with relation to the web entering the main guide roller to a point adjacent a plane through the web edge sensor and normal to the plane of motion of the main web guide roller.
 3. A web guide apparatus according to claim 2 wherein said main web guide roller during tilting movement thereof is disposed tangent to an arcuate path having an arc center located in advance of the main web guide roller and generally perpendicular to the plane of the web entering the main guide roller.
 4. A web guide apparatus according to claim 3 including an entering web guide roller extending transverse to the web in rolling engagement therewith adjacent said arc center and connected to said main web guide roller to move therewith about said arc center.
 5. A web guide apparatus including a main web guide roller extending transverse to a traveling web, means including a movable support mounting said main web guide roller for movement in a plane of motion in which the main web guide roller is disposed generally tangent to a curved path, the effective center of which curved path is spaced from that side of main web guide roller at which the web enters the main web guide roller, at least one web edge sensor mounted to sense the lateral position of the edge of the web entering the main web guide roller at a location in advance of the main web guide roller, and an exiting web guide roller mounted on said movable support for movement with the main web guide roller and at a position offset relative to the main web guide roller in a direction toward the effective center of said curved path and such that the web passing from the main web guide roller and over the exiting web guide roller doubles back and exits tangentially from the exiting roller in an exiting plane, the projection of which exiting plane extends approximately through said location of the web edge sensor.
 6. A web guide apparatus according to claim 5 wherein said plane of motion is generally parallel to the plane of the web entering said main web guide roller.
 7. A web guide apparatus according to claim 5 including an entering web guide roller mounted on said movable support adjacent said effective center of said curved path for movement, said entering web guide roller being disposed in rolling engagement with the web and supporting the web entering said main web guide roller in a plane generally paralleling said plane of motion.
 8. A method of laterally guiding a moving web comprising, passing the web over a main web guide roller tiltable in a plane of motion from a neutral position in which the main web guide roller is perpendicular to the desired path of web travel, sensing the lateral position of the web entering the main web guide roller at a sensing location in advance of the main web guide roller, tIlting the main web guide roller in a direction to correct the deviation of the web from the desired path of travel, causing the web passing from the main web guide roller to double back over an exiting web guide roller and exit tangentially from the exiting web guide roller in an exiting plane, the projection of which exiting plane extends approximately through said sensing location, and maintaining the exiting web guide roller parallel to the main web guide roller during tilting of the latter.
 9. The method of claim 8 including guiding the web entering the main web guide roller in a plane generally parallel to the plane of motion of the main web guide roller. 